Integrated Optical Vapor Cell Apparatus for Precision Spectroscopy

ABSTRACT

An optical waveguide is provided comprising a non-solid core layer surrounded by a solid-state material, wherein light can be transmitted with low loss through the non-solid core layer. A vapor reservoir is in communication with the optical waveguide. One implementation of the invention employs a monolithically integrated vapor cell, e.g., an alkali vapor cell, using anti-resonant reflecting optical waveguides, or ARROW waveguides, on a substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/161,748, filed Jun. 16, 2011, which is a divisional of U.S. patentapplication Ser. No. 12/061,165, filed Apr. 2, 2008, now U.S. Pat. No.8,005,332, issued Aug. 23, 2011, which claims the benefit of U.S.Provisional Application No. 60/917,030, filed May 9, 2007, the entirecontents of which are hereby incorporated by reference in theirentireties.

GOVERNMENT RIGHTS

This invention was made by government support of Grant No. ECS-0500602from The National Science Foundation and Grant No. FA9550-05-1-0432 fromThe Air Force Office of Scientific Research. The Government has certainrights in this invention.

TECHNICAL FIELD

The present invention relates generally to the field of integratedoptics and saturation absorption atomic or molecular spectroscopy on asubstrate, frequency references, or atomic clocks, utilizing an opticalwaveguide comprising a non-solid core layer surrounded by a solid-statematerial, wherein light can be transmitted with low loss through thenon-solid core layer. A vapor reservoir is in communication with theoptical waveguide. One implementation of the invention employs amonolithically integrated vapor cell, e.g., an alkali vapor cell, invapor communication with anti-resonant reflecting optical waveguides,known as ARROWs or ARROW waveguides on a substrate.

BACKGROUND

Over the last few years, it has become possible to confine and guidelight in micrometer-scale hollow-core waveguides based on photoniccrystal structures like photonic crystal fiber (HC-PCF) (Russell, P.,Laser Focus World 38: 77-82, 2002), omniguides (Fink, Y. et al. Science282: 1679-1682, 1998), and Bragg waveguides (Hadley, et al., Opt. Lett.29: 809-811, 2004). Benefits of this approach in the case of gas andvapor phase media include the miniaturization and simplification ofexisting measurement apparatuses, and—perhaps even more significant—theprospect of adding integrated optical components developed for all-solidphotonics to implement new functionalities. Nonlinear optical devicesare particularly attractive because the use of a waveguide eliminatesthe tradeoff between small beam areas and finite focal depth. Thisallows large intensities to be maintained over long distances.Consequently, there are numerous potential applications of hollow-corewaveguide based atomic and molecular spectroscopy, including gas phasesensing, precision spectroscopy (Hansch, T W. et al., Phil. Trans. RoyalSoc. London A 363: 2155-2163, 2005), atomic clocks (Knappe, S. et al.Opt. Lett. 30: 2351-2353, 2005), nonlinear frequency generation(Benabid, et al., Phys. Rev. Lett. 95, 213903, 2005), low-levelall-optical switching (Dawes, et al., Science 308, 672-674, 2005), slowlight (Lukin, MD. Rev Mod Phys 75:457-72, 2003; Hau, et al., Nature397:594-598, 1999), and quantum communications (Eisaman MD. et al.,Nature, 438: 837-841, 2005; Kolchin, et al., Phys. Rev. Lett. 97:113602, 2006). The latter areas are examples of the use ofelectromagnetically induced transparency (EIT) (Harris, S E., Phys.Today 50: 36-42, 1997)—extremely strong linear and nonlinearlight-matter interactions that result from quantum interference effects.Alkali metal vapors are ideally suited for EIT as well as for many otherapplications, making integrated rubidium or cesium cells highlydesirable. Up to now, most work in the area of confined gas spectroscopyhas been carried out with cylindrical photonic crystal (HC-PCF) fiber.Confinement and spectroscopy of gases (Benabid et al., Nature434:488-491, 2005; Ghosh et al., Phys. Rev. Lett. 94: 093902, 2005),generation of nonlinear amplification (Benabid, et al., Phys. Rev. Lett.95, 213903, 2005), EIT and saturated absorption spectroscopy inacetylene (Ghosh et al., Phys. Rev. Lett. 94: 093902, 2005; Couny etal., Opt. Comm. 263: 28-31, 2006; Thapa et al. Opt. Lett. 31: 2489-2491,2006), and signatures of quantum interference in rubidium vapor (Ghoshet al., Phys. Rev. Lett. 97: 023603, 2006) have been demonstrated andare indicative of the promise and rapid progress in this field. TheHC-PCF-based approach has many advantages, in particular low waveguideloss and resulting long interaction lengths. However, it also haslimitations such as the current requirement for attaching rubidiumreservoirs connected to a vacuum pump system at the open ends of theHC-PCF (Ghosh et al., Phys. Rev. Lett. 97: 023603, 2006) therebypreventing full integration and leaving the complete apparatus large.Another characteristic is the restriction of optical confinement andinteraction to one dimension.

Guiding light through hollow optical waveguides has opened photonics toinvestigating non-solid materials with the convenience of integratedoptics. Of particular interest is the confinement of atomic vapors suchas alkali vapors, due to the wide range of applications including slowand stopped light (Lukin, MD, Rev Mod Phys 75:457-72, 2003),single-photon nonlinear optics (Schmidt, H. and Imanoglu, A., Opt Lett21:1936-1938, 1996), quantum information processing (Eisaman MD et al.,Nature, 438: 837-841, 2005), precision spectroscopy (Hansch, T. W. etal., Phil. Trans. Royal Soc. London A 363: 2155-2163, 2005), andfrequency stabilization (Danielli, et al., Opt. Lett. 25: 905-907,2000). A need exists in the art for an integrated platform to enableprecision atomic or molecular spectroscopy that combines the advantagesof photonic crystal-like structures with integrated optics.

SUMMARY

The present invention relates generally to the field of integratedoptics and saturation absorption atomic or molecular spectroscopy on asubstrate, frequency references, or atomic clocks, utilizing an opticalwaveguide and an optical measurement system. The optical waveguide cancomprise a non-solid core layer surrounded by a solid-state material,wherein light and an atomic or molecular vapor can be confined andtransmitted with low loss through the non-solid core layer. A vaporreservoir is in communication with the optical waveguide. Oneimplementation of the invention employs a monolithically integratedvapor cell, e.g., an alkali vapor cell, using anti-resonant reflectingoptical waveguides, ARROWs or ARROW waveguides, on a substrate.

An optical waveguide is provided which comprises a substrate made of asolid material and multiple layers of solid state material disposed onthe substrate; a non-solid core extending through at least one of saidmultiple layers, whereby said non-solid core may be used to contain asample material; a perpendicular waveguide portion for use in injectinglight into said non-solid core; and a vapor reservoir for use incontaining a vapor in communication with said non-solid core; whereinsaid multiple layers of solid state material are constructed to formanti-resonant reflecting layers adjacent to said non-solid core, wherebylight is substantially prevented from leaking out of said core in atransverse direction. The vapor can be an alkali vapor, e.g., rubidium,cesium, or sodium, an elemental vapor, e.g., barium, or a molecularvapor, e.g., iodine, HCN, or acetylene. An optical measurement system ora planar atomic or molecular spectroscopy system are provided comprisingthe optical waveguide. A system for making parallel optical measurementsis provided comprising an optical waveguide in one or more parallelchannels within a solid state material.

An optical waveguide is provided which comprises a substrate made of asolid material and multiple layers of solid state material disposed onthe substrate; a non-solid core extending through at least one of saidmultiple layers, whereby said non-solid core may be used to contain avapor; an intersecting waveguide portion for use in injecting light intosaid non-solid core; and a vapor reservoir for use in containing thevapor in communication with said non-solid core; wherein said multiplelayers of solid state material are constructed to form anti-resonantreflecting layers adjacent to said non-solid core, whereby light issubstantially prevented from leaking out of said core in a transversedirection. The vapor can be an alkali vapor, e.g., rubidium, cesium, orsodium, an elemental vapor, e.g., barium, or a molecular vapor, e.g.,iodine, HCN, or acetylene. The substrate can comprise Silicon (Si) andthe multiple layers can comprise SiO.sub.2 and SiN. In one aspect, thenon-solid core has an index of refraction which is lower than the indexof refraction of the surrounding solid-state material, and wherein lightcan be transmitted with low loss through the non-solid core. Theintersecting waveguide portion can be configured to permit transmissionof counterpropagating light in two or more directions through thechannel to create narrow spectral features. The spectral features canresult from transmission, absorption, or interference. The intersectingwaveguide portion can be substantially perpendicular to said non-solidcore. The intersecting waveguide portion can adjoin the channel at anangle between 0° and 180°. In one aspect, the intersecting waveguideportion can be substantially linear to said non-solid core. In a furtheraspect, the reservoir is sealed to said substrate by anodic bonding,epoxy, or solder, and the seal is an airtight seal. The reservoir can besealed, for example, with an o-ring seal, a screw fitting, or bycrimping at the end of the reservoir. The reservoir can be glass ormetal, for example, copper. A vapor source can be in a form of solid,liquid or gas.

An optical waveguide generally structured as an anti-resonant reflectingoptical waveguide (ARROW) is provided which comprises a substrate andmultiple layers of solid state material, including SiO₂ and SiN,disposed on the substrate, and a non-solid core extending through atleast one of said multiple layers, wherein said non-solid core has anindex of refraction which is lower than the index of refraction of thesurrounding solid-state material, and wherein light can be transmittedwith low loss through the non-solid core; a multilayer reflector, e.g.,a Fabry-Perot reflector or a Bragg reflector, adjacent to said non-solidcore, for substantially preventing light from leaking out of said corein a transverse direction; an intersecting waveguide portion for use ininjecting light into said non-solid core; a vapor reservoir for use incontaining a vapor in communication with said non-solid core; andwhereby said non-solid core may be used to contain said vapor whoseoptical properties are to be measured. Optical properties include, butare not limited to, light transmission, light absorption, interference,photon detection, or photon generation. In one aspect, the injectedlight is used for measuring absorption characteristics associated withsaid vapor. The intersecting waveguide can adjoin the channel at anangle between 0° and 180°. In one aspect, the intersecting waveguideportion can be substantially perpendicular to said non-solid core. Theintersecting waveguide portion can be substantially linear to saidnon-solid core. The intersecting waveguide portion can be configured topermit transmission of counterpropagating light in two or moredirections through the channel to create narrow spectral features. Thespectral features can result from transmission, absorption, orinterference.

An optical measurement system is provided which comprises an opticalwaveguide comprising a channel surrounded by a solid-state material,including a multilayer reflector, e.g., a Fabry-Perot reflector or aBragg reflector, adjacent to said channel; a vapor reservoir for use incontaining a vapor in communication with said non-solid core; and anintersecting waveguide portion for use in injecting light into thechannel, wherein the intersecting waveguide portion is configured topermit transmission of counterpropagating light in two or moredirections through the channel to create narrow spectral features. Thespectral features can result from transmission, absorption, orinterference. The intersecting waveguide can adjoin the channel at anangle between 0° and 180°. A structure for injecting light into saidchannel is provided wherein the injected light is guided within thechannel and through said vapor, and a device for measuring selectedoptical properties associated with said vapor, wherein said selectedoptical properties include light transmission, light absorption,interference, photon detection, or photon generation associated withsaid vapor over macroscopic distances within the channel.

A system for making parallel optical measurements, comprising an opticalwaveguide comprising a generally planar solid-state material and one ormore non-solid parallel channels within said solid-state material,including a multilayer reflector, e.g., a Fabry-Perot reflector or aBragg reflector, adjacent to each channel, whereby light injected intosaid channels is substantially prevented from leaking out of saidchannels in a transverse direction; a vapor reservoir for use incontaining a vapor in communication with said one or more parallelchannels; and one or more intersecting waveguide portions for use ininjecting light into the channels; and means for measuring selectedoptical properties associated with said vapor. The intersectingwaveguide portion can be configured to permit transmission ofcounterpropagating light in two or more directions through the channelto create narrow spectral features. The spectral features can resultfrom transmission, absorption, or interference. The system can be usedfor saturation absorption spectroscopy, frequency reference, or anatomic clock.

A planar atomic or molecular spectroscopy system is provided whichcomprises an optical waveguide comprising a channel surrounded by asolid-state material, including a multilayer reflector, e.g., aFabry-Perot reflector or a Bragg reflector, adjacent to said channel,wherein said channel is configured for low loss at a first opticalwavelength and high loss at a second optical wavelength; a vaporreservoir for use in containing a vapor in communication with saidchannel; and an intersecting waveguide portion for use in injectinglight into the channel, wherein the intersecting waveguide portion isconfigured to permit transmission of counterpropagating light in two ormore directions through the channel to create narrow spectral features.The spectral features can result from transmission, absorption, orinterference. The system can be used for saturation absorptionspectroscopy, frequency reference, or an atomic clock.

Other features and advantages of the present invention are describedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent or application contains at least onedrawing/photograph executed in color. Copies of this patent or patentapplication publication with color drawings will be provided by theOffice upon request and payment of the necessary fee.

FIGS. 1( a)-1(c) show a planar atomic spectroscopy chip.

FIG. 2 shows rubidium spectroscopy on a chip.

FIGS. 3( a)-(b) show characteristics of an integrated rubidium cell.

FIGS. 4( a)-4(b) show atomic spectroscopy on a chip and characteristicsof an integrated ARROW Rb cell.

FIGS. 5( a)-5(c) show fabrication of an integrated rubidium cell.

FIG. 6 shows a rubidium spectroscopy setup.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention relates to an apparatus or system in integratedoptics on a substrate which can be utilized in accurate measurements ofatomic or molecular spectroscopy, for example, saturated absorptionatomic spectroscopy. A system is provided which utilizes an opticalwaveguide comprising a non-solid core layer surrounded by a solid-statematerial, wherein light and atomic or molecular vapor can be transmittedwith low loss through the non-solid core layer. A vapor reservoir is incommunication with the non-solid core of the optical waveguide. Oneimplementation of the invention employs a monolithically integratedatomic or molecular vapor cell, e.g., an alkali vapor cell, usinganti-resonant reflecting optical waveguides, ARROWs or ARROW waveguides,on a substrate.

The present invention provides a monolithically integrated atomic ormolecular vapor cell using hollow-core antiresonant reflecting opticalwaveguides (ARROWs) on a substrate. The cells have a footprint below 1cm², fully planar fiber-optical access, and a cell volume more thanseven orders of magnitude less than conventional bulk cells. Themicron-sized mode areas enable high beam intensities overnear-centimeter lengths. Optical densities have been demonstrated inexcess of 2 and saturation absorption spectroscopy on a chip. Theseresults enable the study of atoms and molecules on a platform thatcombines the advantages of photonic crystal-like structures withintegrated optics.

In an embodiment of the invention, a monolithically integrated, planarrubidium vapor cell on a substrate is provided. We experimentallydemonstrate the key requirements for integrated atomic spectroscopy,including confinement of both light and rubidium vapor in micron-scaledhollow-core optical waveguides, e.g., ARROW waveguides, large opticaldensity over chip-scale distances, high intensities for efficientnonlinear effects, and fiber optics based saturation absorptionspectroscopy. In addition, we demonstrate that the design of our atomicspectroscopy platform can contain functional features that cannot beimplemented by using photonic crystal fibers.

This approach provides advantages compared to state-of-the-arttechniques:

-   -   low-loss guiding of light inside a narrow channel of low-index        media (gaseous or liquid) on a semiconductor chip. Low-index in        this context means that the refractive index of the sample        material is less than any of the indices of the solid-state host        material.    -   Ability to guide light in the same volume as the low-index        material. This allows for transmission, absorption or        interference measurements over macroscopic distances.    -   Ability to discriminate/filter selective wavelengths along the        sample volume. This results from the fact that the waveguide is        optimized for a desired wavelength range.    -   Entirely planar technology for high sensitivity optical        measurements compatible with fiber-optic technology.    -   Massive parallelism for multiple measurements on a single chip.    -   Potential for further integration with additional optical        elements such as photo detectors on the same chip.    -   Ability for optical measurements on microchannels of an order of        magnitude smaller dimension.    -   Specific methods to fabricate hollow-core ARROW waveguides based        on sacrificial core layers.    -   Platform for realizing large nonlinear phase shifts between        light signals using EIT in atoms, e.g., Rb.    -   Integrated platform for saturation spectroscopy and frequency        stabilization.    -   Fiber-optic coupling.    -   Automatic alignment of counter-propagating beam by the optical        waveguide for optimized interaction.    -   Can be integrated with other optical/electrical elements on a        chip.    -   Possibility to add integrated temperature control or magnetic        fields on the chip.    -   Possibility to achieve local control over temperature or        magnetic fields via microstructuring of the underlying chip.

An atomic or molecular vapor cell in communication with an opticalwaveguide can be used for specific applications, for example, frequencystabilization or atomic or molecular references. The waveguide can beloaded either by atomic/molecular diffusion or by heating the chip,wherein the vapors or gases move into the non solid core of thewaveguide.

An intersecting solid waveguide portion can inject light into thenon-solid core waveguide. The intersection of the waveguides can besubstantially perpendicular or substantially in the linear direction, orat various intermediate angles of intersection, for example, at an anglebetween 0° and 180°. A substantially linear intersection allows theautomatic alignment of counterpropagating beams which is important forfrequency stabilization as well as fiber optic coupling of light intothe non-solid core waveguide.

Vapor reservoirs containing vapor sources such as rubidium are attachedto the substrate by an air-tight seal. Reservoirs can be made frommaterials, including but not limited to, glass or metal. The reservoircan be attached to the chip through anodic bonding, epoxies, or soldersto provide an air-tight seal between the substrate and the reservoir.The reservoirs can be placed over an opening at the end of the waveguideso that vapor contained in the reservoir can fill the hollow waveguide.Sources for the vapor include solids, for example, rubidium or cesiumplaced in the reservoir, liquids placed into the reservoir, or gasesthat were injected into the reservoir. After inserting a solid, liquid,or gas vapor source, the reservoir can be sealed using epoxy, a metalcompression fitting, soldering, welding, anodic bonding, heating orcrimping the end of the reservoir. Furthermore, an o-ring seal or ascrew fitting can be used to seal the reservoir.

One goal of ours is to have highly functional, highly parallelstructures naturally combined with other integrated elements such asinterferometers and detectors on the same chip. The research describedherein provides a demonstration of waveguiding of light and/or atomic ormolecular vapors in ARROW structures with gaseous or liquid core layersproviding a wide range of applications including slow and stopped light,single-photon nonlinear optics, quantum information processing,precision spectroscopy, and frequency stabilization.

As a result of our research, better measurement tools will evolve thatwill improve both our fundamental understanding of health-relatedprocesses and physical measurement techniques as well as lead toimproved flow cytometry methods.

Below we provide a more detailed description of exemplary embodimentsand applications of the present invention. The focus can be on fluidapplications, as well as applications to gases. In addition, it shouldbe noted that invention may be carried out with a variety of substrateand waveguide materials, including the materials discussed in connectionwith the examples described below as well as those listed below (thislist is not intended to be exhaustive).

Exemplary substrates are provided:

-   -   Semiconductors (useful for integrating electronic and        optoelectronic devices (III-V semiconductors) with the        waveguide), including silicon, Ge, diamond, all III-V        semiconductors (GaAs, InP, HgCdTe, GaN, GaP, etc.).    -   Metals (useful for making a low cost device), including Al, Tin,        Titanium, Copper, etc.    -   Plastics and Polymers (again useful for a low cost device and        integrating with electronics on PCB boards). Insulators like        ceramic or glass (useful because they produce a transparent        substrate or because of heat mitigation).    -   Silicon based glasses—silica, quartz, soda lime, boron doped,        etc.    -   alumina, sapphire

Exemplary waveguide materials:

-   -   Any material possibly deposited by chemical vapor deposition,        including silicon dioxide, silicon nitride, silicon oxy-nitride        (important because they are very commonly deposited by chemical        vapor deposition).    -   Any material that could be sputtered or evaporated onto a        substrate, including silicon dioxide, silicon nitride, and        silicon-oxynitride.    -   Any material that could be spun-on or dip coated including        spin-on-glass, polyimides, and polymer based materials.

Exemplary sacrificial layer materials:

-   -   Any metal, including aluminum, silver, gold, titanium, tungsten,        copper.    -   Polymer materials, including SUB, photoresist, and polyimide.

DETAILED DESCRIPTION OF EXEMPLARY IMPLEMENTATIONS AND APPLICATIONS

We will now explain our invention in sufficient detail to enable aperson of ordinary skill in the field of integrated optics to make anduse the invention without undue experimentation. The followingdescription is not intended (nor would it be possible) to serve as anexhaustive discussion of every possible embodiment, application ormethod of manufacturing a device within the scope of our invention. Itis sufficient, however, to enable the skilled artisan to practice ourinvention. We will first briefly discuss our preliminary studies andthen we will explain a method for fabricating exemplary embodiments ofthe invention, optical measurements for characterization and testing, anintegrated rubidium cell and saturation absorption atomic spectroscopyutilizing an integrated ARROW rubidium vapor cell.

Nonlinear optical devices, such as ARROW waveguide atomic or molecularvapor cells, are particularly attractive because the use of a waveguideeliminates the tradeoff between small beam areas and finite focal depth.This allows large intensities to be maintained over long distances.Consequently, there are numerous potential applications of hollow-corewaveguide based atomic and molecular spectroscopy, including gas phasesensing, precision spectroscopy (Hänsch, T W. et al., Phil. Trans. RoyalSoc. London A 363: 2155-2163, 2005), atomic clocks (Knappe, S. et al.,Opt. Lett. 30: 2351-2353, 2005), nonlinear frequency generation(Benabid, et al., Phys. Rev. Lett. 95, 213903, 2005), low-levelall-optical switching (Dawes, et al., Science 308, 672-674, 20050, slowlight (Lukin, MD, Rev Mod Phys 75:457-72, 2003; Hau, et al., Nature397:594-598, 1999), and quantum communications (Eisaman MD, et al.,Nature, 438: 837-841, 2005; Kolchin, et al., Phys. Rev. Lett. 97:113602, 2006). The latter areas are examples of the use ofelectromagnetically induced transparency (EIT) (Harris, S. E., Phys.Today 50: 36-42, 1997)—extremely strong linear and nonlinearlight-matter interactions that result from quantum interference effects.Alkali metal vapors are ideally suited for EIT as well as for many otherapplications, making integrated rubidium or cesium cells highlydesirable.

In an embodiment of the invention, a monolithically integrated rubidiumvapor cell is provided using hollow-core antiresonant reflecting opticalwaveguides (ARROWs) on a silicon chip. The cells have a footprint below1 cm², fully planar fiber-optical access, and a cell volume more thanseven orders of magnitude less than conventional bulk cells. Themicron-sized mode areas enable high beam intensities overnear-centimeter lengths. We demonstrate optical densities in excess of 2and saturation absorption spectroscopy on a chip. These results enablethe study of atoms and molecules on a platform that combines theadvantages of photonic crystal-like structures with integrated optics.

FIG. 1( a) shows a schematic overview of the planar integrated atomicspectroscopy chip (rubidium cell). Optical signals are guided on thechip using both hollow and solid core antiresonant reflecting opticalwaveguides (ARROWs). ARROWs are akin to photonic crystal or Braggstructures in that they rely on the use of dielectric layers withappropriate thicknesses for confinement of light (Duguay, et al., Appl.Phys. Lett. 49: 13-15, 1986). They do not, however, have to be based onperiodic structures which result in a waveguide description based onphotonic bands and Bragg wave vectors. It has been shown recently thatquasi single mode hollow core waveguides for both air and liquids can bebuilt on a chip using the ARROW principle which requires fulfilling theantiresonance condition for the transverse wave vector component in eachdielectric layer (Yin, et al., Opt. Express, 12: 2710-2715, 2004). It isimportant to note that with proper layer design the hollow-corewaveguides can be interfaced with solid core ARROWs as shown in thelongitudinal cross section in FIG. 1B. These connections allow forsimultaneous efficient light guiding along the hollow core andtransmission between solid and hollow waveguides (Schmidt et al., IEEEJ. of Selected Topics in Quantum Electronics, 11: 519-527, 2005) and hasbeen used to demonstrate single molecule detection sensitivity in liquidcores (Yin et al., Opt. Lett. 31, 2136-2138, 2006). This capability isreflected by the design in FIG. 1( a) and is essential for creatingself-contained optical volumes with sealed connections to the edges of achip. It also enables two-dimensional waveguide networks which cannoteasily be created with photonic crystal fibers. FIG. 1( a) also depictshow the vapor of interest is introduced into the waveguides. Thehollow-core ARROWs have two open ends. A reservoir containing solidrubidium is placed over one end, and the other end is sealed. Theoptical beam path is then filled with rubidium atoms due to rubidium'shigh vapor pressure. The cell is completely self-contained and no vacuumapparatus is required to interface the reservoirs. Moreover, thereservoirs are placed in a location where they do not interfere with theoptical beam path. FIG. 1 c is a photograph of an ARROW-chip that wasfabricated using this method. The figure shows two Rb reservoirs inparallel on the same chip, illustrating one advantage of the planarintegrated approach—realization of several independent cells on a singledevice in a small volume.

The waveguide structures shown in FIG. 1( a)-(c) are formed using apreviously described fabrication process (Barber J P. et al. IEEE Phot.Tech. Lett. 17: 363-365, 2005). The confinement layers are composed ofplasma-enhanced chemical vapor deposited silicon nitride and oxide, andthe hollow core is created by a sacrificial etching process (see Methodssection for details). The reservoirs are attached and filled withnatural rubidium in an inert helium atmosphere. Sealing to the chip isprovided by epoxy adhesive and a metal screw on top of the reservoir(see FIG. 5 for details). It is important to note that the reservoirsare not part of the optical beam path and do not impact the opticalproperties of the ARROW-based cell. The waveguides were designed for lowloss across the Rb D-lines (780 to 795 nm) in solid and hollow cores andtransmissive connections between them using the design principlesdescribed in ref. 22.

FIG. 1( a)-(c) shows a planar atomic spectroscopy chip. a. Layoutshowing interconnected hollow-core (HC) and solid-core (SC) ARROWwaveguides to form two independent vapor cells on a chip connected byadditional SC waveguide. Sealed, rubidium-filled reservoirs are attachedat the open ends of the HC waveguides. Fiber-optical access and beampath across the chip are shown for the lower vapor cell. b. Crosssection along the HC-WG length showing the ARROW confinement layers(light grey: SiO₂, dark grey: SiN), beam path, and rubidium atoms insideHC-ARROW. c. Fabricated ARROW-based atomic spectroscopy chip (layout asin FIG. 1( a) but rotated clockwise by 90°.). The optical beam path (redarrow) in the hollow core is 5 mm.

For the following optical experiments, the chips were mounted on awaveguide translation stage. Light from tunable external cavity diodelasers was coupled into the solid-core ARROW as shown in FIG. 1( a)using end-coupling from a single-mode fiber. Transmitted light wascollected with an objective lens, focused on a photodetector, andrecorded as a function of wavelength (see Methods section and FIG. 6 fordetails). Alternatively, the excitation beam could be directed to aconventional bulk rubidium reference cell. FIG. 2 shows the normalizedhyperfine-split absorption spectra of the Rb D2-line around 780 nm takenat a temperature of 70° C. for both bulk and integrated ARROW rubidiumcells. Clearly, the integrated Rb cell shows a clean absorption signalthat is nearly identical to that of the bulk cell, demonstrating theessential functionalities of the ARROW chip: confinement of bothrubidium atoms and light within the same hollow-core waveguide. Theblack lines represent Gaussian fits to the spectra and are nearlyindistinguishable from the data for both bulk and ARROW cell. A detailedanalysis of the absorption spectrum and the broadening mechanisms willbe presented elsewhere. Also shown in the figure is a mode image of thehollow-core mode taken on a sample with open hollow ends but identicalcore dimensions of 5×12 microns. The FWHM mode area in the hollow coreis only 14λ²=8.8 μm², showing that it is possible to confine light toareas comparable to atomic cross sections over macroscopic distances ona chip. For comparison, the focal depth of a Gaussian beam of the samearea would only be 22 μm, or a factor 230 shorter than the 5 mmpropagation distance in the ARROW cell. As discussed in theintroduction, the tight confinement over orders of magnitude longerlengths is particularly beneficial for nonlinear optical effects.

FIG. 2 shows rubidium spectroscopy on a chip. Normalized hyperfineabsorption spectrum of natural rubidium D2 line for bulk (top) andintegrated ARROW cell (bottom) at 70° C. The leftmost peak arises from.⁸⁷Rb (5S_(1/2)(F=2)→5P_(3/2)), the other peaks from ⁸⁵Rb(5S_(1/2)(F=2)→>5P_(3/2)), and 5S_(1/2)(F=3)→5P_(3/2)). Each peakcontains contributions from three transitions that are not resolved dueto Doppler broadening. Black lines: Fits with Gaussian absorptionprofiles. Inset: ARROW waveguide mode image recorded with CCD camera.

A key metric for the usefulness of an integrated rubidium cell, inaddition to the ability to observe a guided mode and an absorptionspectrum, are the levels of both atomic and optical density that can beachieved. In particular, it has been shown that EIT-based nonlineareffects require optical densities in excess of one to be truly practical(Lukin, M D. and Imamoglu, A, Nature 413: 273-276, 2001). In FIG. 3( a),we show the atomic densities that were extracted from the measuredabsorption profiles for both bulk and ARROW cells as a function oftemperature. We find that density levels are similar in both casesalthough the integrated cell has larger deviations at lower temperatureswhere the optical signal from the ARROW cell is relatively noisy. Thelow temperature discrepancies likely stem from measurement uncertaintiesand atomic adhesion to the ARROW walls. FIG. 3B shows the correspondingoptical density in the ARROW cell obtained from the ⁸⁷Rb(5S_(1/2)(F=1)→>5P_(3/2)) absorption peak. We find that for temperaturesabove 85° C., an optically dense medium can be created for this weakestD2 transition, indicating that above this temperature the vapor isoptically thick across the entire line. A maximum value of 2.21 at 95°C. is observed for this peak which demonstrates that the integratedARROW cell is a promising candidate for nonlinear quantum coherenceeffects on a chip. Higher optical densities can be achieved with longerhollow-core sections and by increasing the atomic density, e.g. usinglight induced desorption (Alexandrov et al., Phys. Rev. A 66: 042903,2002).

FIG. 3( a)-3(b) shows characteristics of integrated rubidium cell. a.Atomic density versus temperature for bulk (open squares) and integratedARROW cell (circles). b. Optical density in integrated ARROW cell versustemperature for the weakest ⁸⁷Rb (5S_(1/2)(F=1)→>5P_(3/2)) transition.An optically dense vapor (OD=α.L>1) is observed above 85° C.

Finally, we investigated the suitability of the ARROW cell for commonprecision spectroscopy applications. The measurement setup was modifiedto accommodate coupling light from both ends into the chip todemonstrate saturated absorption spectroscopy (SAS) as detailed in theMethods section and FIG. 6. SAS is a commonly used method for frequencystabilization of a light source by locking its emission frequency tothat of an atomic transition (Danielli et al., Opt. Lett. 25: 905-907,2000). Typically, counterpropagating beams in bulk atomic vapor cellsare used to create narrow spectral features by the elimination ofDoppler broadening due to selection of atoms with zero velocity relativeto the beams. The width of these Lamb dips is determined by the muchsmaller homogeneous linewidth of the transition and leads to moreaccurate references. Normally, overlapping the strong pump with theweaker probe beam requires some alignment effort. In our integratedARROW cell, however, this alignment is automatically accomplished by theoptical waveguides, demonstrating another advantage of the use ofintegrated optical elements. FIG. 4( a)-4(b) shows the SAS spectrumobserved in the ARROW cell and clearly shows the characteristic Lambdips. A width of approximately 34±2 MHz is observed which is consistentwith additional homogeneous transit-time broadening.

We have presented a new type of integrated optical platform for atomicspectroscopy on a chip based on ARROW waveguides and demonstrated theessential functionalities as required for various applications, inparticular nonlinear optics. The table in FIG. 4B summarizes the mainproperties of the ARROW-based rubidium cell in comparison withconventional bulk cells. The improvements afforded by the ARROW in termsof intensities and volume, range from over four to over seven orders ofmagnitude. We also compare its characteristics to cesium minicells thathave recently been developed at NIST (Liew et al., Appl. Phys. Lett. 84:2694-2696, 2004; Knappe, S. et al. Opt. Lett. 30: 2351-2353, 2005).While both are compact and excellent candidates for miniaturized atomicclocks, the optical ARROW cell volume and mode areas are orders ofmagnitude smaller and allows for much higher intensities, mainly due tothe novel ability to guide light through the atomic medium on a chip.The integrated ARROW cells can be interfaced with conventional fiber andhave immediate near-term applications, for example portable referencecells for frequency stabilization as demonstrated by the SAS experiment.Compared with HC-PCF fiber rubidium cells, the longer interaction pathin a HC-PCF offers advantages for increasing the delay time in slowlight experiments and for spectroscopy of molecular gases such asacetylene. For many applications, however, path lengths on the order ofcentimeters as in the ARROW cell are sufficient. For example, the largeoscillator strengths of atoms means that only chip-scale distances arerequired to achieve high optical densities as demonstrated in FIG. 3B.Another example where short path lengths are beneficial isimplementation of nonlinear optical effects such as giant Kerrnonlinearities (Schmidt, H. & Hawkins, A. R.,. AppL Phys. Lett. 86:032106, 2005) that are dispersion rather than absorption limited. Inaddition to the advantages illustrated in FIG. 1( c) (more compact totalsize and the straightforward definition of multiple cells on a singlechip), additional unique possibilities in the ARROW cell arise from theoption to add waveguides (See FIG. 1) that intersect the width of thecell. This could, for instance, be used to carry out sub-Dopplerspectroscopy in thin vapor films (Briaudeau et al., Phys. Rev. A 59:3723-3735, 1999) on a chip. The application of these cells toimplementing quantum interference effects such as slow light or singlephoton nonlinearities on a chip presents a very intriguing and excitingpath. In order to be able to fully utilize the quantum coherencephenomena, dephasing of the coherence between atomic levels must beavoided. This is challenging in tightly confined cells, but it was shownboth theoretically (Schmidt, H. & Hawkins, A. R.,. Appl. Phys. Lett. 86:032106, 2005) and experimentally in photonic crystal fibers (Ghosh etal., Phys. Rev. Lett. 97: 023603, 2006) that the dephasing can besufficiently compensated by suitable organic wall coatings. Theintersecting waveguide geometry is perfectly suited for quantumcoherence based nonlinear single photon generation as it allows thecollection and detection of photons generated in a degenerate parametricprocess by the solid core waveguides (Kolchin, et al., Phys. Rev. Lett.97: 113602, 2006). In preliminary experiments, we have already observedRb absorption in the ARROW cell after scattered light was coupled intothe Rb cell via an intersecting solid core waveguide. In addition,different gases, functional waveguide geometries, and optical elementssuch as DBR gratings can be used on the ARROW platform, and some of thetechniques described here may even be applied to nanophotonic structuressuch as slot waveguides (Xu et al., Opt. Lett. 29:1626-1628, 2004). Allthese options will stimulate further developments and new applicationsfor atomic spectroscopy on a chip.

FIG. 4( a)-4(b) shows atomic spectroscopy on a chip and characteristicsof integrated ARROW Rb cell. a. Saturation absorption spectroscopy (SAS)using counterpropagating beams in integrated ARROW cell. The grey arrowsmark the Lamb dips resulting from the elimination of Doppler broadening.b. Comparison of main characteristics of integrated ARROW cell withconventional bulk vapor cell.

FIGS. 5( a)-5(c) shows fabrication of integrated rubidium cell. a. Chipafter waveguide fabrication and opening ends of hollow core. b.Attachment of Rb reservoir (left) and epoxy seal (right) over openhollow-core waveguide ends. c. Addition of rubidium and sealing ofreservoir in helium atmosphere.

Methods

Atomic spectroscopy chip fabrication. Hollow-core waveguides were builtwith a previously described process (Barber J P. et al. IEEE Phot. Tech.Lett. 17: 363-365, 2005) combining plasma-enhanced deposition ofdielectric layers (SiO₂ and SiN) and patterning of a sacrificial corelayer (SU-8; Microchem, Newton, Mass.). Solid-core waveguides forexcitation and collection were formed by a photolithography step todefine the solid-core ridge with a 1 μm deep reactive ion etch (AnelvaCorp., Japan) in a CF₄ atmosphere. The ends of the SU-8 core wereexposed using the same reactive ion etcher to locally remove the SiO₂and SiN coating layers. The SU-8 core was then removed in a selectivechemical etch (Nanostrip; Rockwood Electronic Materials, Fremont,Calif.). (FIG. 5( a)). FIG. 5( b) shows that after cleaving the chip tothe desired size, a stainless steel standoff was attached over one ofthe open ends with an epoxy adhesive. The other open end was sealed witha drop of epoxy. The chip was then placed in a controlled environmentglovebox (Vacuum Atmospheres, Hawthorne, Calif.) filled with helium.Solid rubidium droplets were then transferred from a glass ampoulesource into the stainless steel standoff The top of the standoff wassealed using a stainless steel screw and butyl rubber o-ring as shown inFIG. 5( c), resulting in an integrated rubidium cell with helium buffergas at atmospheric pressure.

The hollow core dimensions of the waveguide were 5×12 μm, and the solidcore waveguide width was 12 μm. The dielectric layer sequence for theARROW waveguides was starting from the substrate (all values in nm):SiO₂/SiN/SiO₂/SiN/SiO₂/SiN—core—SiN/SiO₂/SiN/SiO₂/SiN/SiO₂(550/110/550/110/550/110/5000/303/216/162/379/139/3402).

Saturated absorption spectroscopy (SAS). The setup for the SASexperiment is shown in FIG. 6. Light from a commercial external cavitydiode laser (ECDL, New Focus) is first split by a polarizing beamsplitter (PBS) into pump and probe beams. The probe is coupled into thewaveguide via single mode optical fiber. The output from the probe beamis collected by an objective lens and monitored by a CCD camera. Once agood mode from the core area of the output facet is observed, therelative positions of objective lens and the waveguide are fixed,ensuring proper alignment of the counterpropagating pump beam. The pumpbeam is directed through a polarization independent beam splitter beforeit is coupled into the waveguide. The polarization direction of theprobe and coupling beam can be controlled by the fiber polarizationcontroller and the half-wave plate before the beam splitter,respectively. The relative power ratio of the probe and coupling beam iscontrolled by the half-wave plate before the first PBS. After both beamsare coupled into the waveguide, the CCD camera is replaced by aphotodetector to record the probe spectrum.

What is claimed:
 1. A method for sealing a vapor reservoir to anintegrated waveguide chip, the method comprising: attaching a first endof the vapor reservoir to the chip; placing an amount of vapor sourceinside the vapor reservoir; establishing a vacuum environment within thevapor reservoir; and sealing a second end of the vapor reservoir.
 2. Themethod of claim 1, wherein the vapor reservoir has a metallic structure.3. The method of claim 1, wherein the vapor reservoir is a coppercylinder.
 4. The method of claim 1, wherein the vapor reservoir is madeof glass.
 5. The method of claim 1, wherein said attaching a first endof the vapor reservoir comprises placing the first end of the vaporreservoir over an opening at a first end of a waveguide comprised in thechip.
 6. The method of claim 5 further comprising using epoxy to seal anopening at a second end of the waveguide.
 7. The method of claim 1,wherein said attaching a first end of the vapor reservoir comprisescreating an airtight seal between the first end of the vapor reservoirand the chip.
 8. The method of claim 7, wherein said creating anairtight seal comprises using at least one member of a group consistingof anodic bond, epoxy, solder, metal compression fit, weld, heat, andcrimp.
 9. The method of claim 1, wherein the vapor source comprisesusing at least one member of a group consisting of alkali vapor,elemental vapor, and molecular vapor.
 10. The method of claim 9, whereinthe alkali vapor arises from an alkali metal element.
 11. The method ofclaim 9, wherein the alkali vapor arises from one member of a groupconsisting of rubidium, cesium, and sodium.
 12. The method of claim 9,wherein the elemental vapor comprises an alkaline earth metal element.13. The method of claim 9, wherein the elemental vapor comprises barium.14. The method of claim 9, wherein the molecular vapor comprises onemember of a group consisting of iodine, acetylene, and hydrogen cyanide.15. The method of claim 1, wherein the vapor source comprises one memberof a group consisting of solid, liquid, and gas source.
 16. The methodof claim 1, wherein said steps of placing an amount of vapor source,establishing a vacuum environment, and sealing a second end of the vaporreservoir are conducted in a controlled environment glovebox.
 17. Themethod of claim 1, wherein said establishing a vacuum environmentcomprises filling the vapor reservoir with an inert gas at atmosphericpressure.
 18. The method of claim 17, wherein the inert gas is helium ornitrogen.
 19. The method of claim 1, wherein said sealing a second endof the vapor reservoir comprises using at least one member of a groupconsisting of o-ring seal, screw fitting, and crimping.
 20. A method forsealing a vapor reservoir to an integrated waveguide chip, the methodcomprising: cleaving the chip to a desired size; attaching a first endof the vapor reservoir over a first open end of a hollow waveguidecomprised in the chip, wherein the vapor reservoir comprises a metalliccylinder, and wherein epoxy is used to form an air-tight seal betweenthe first end of the vapor reservoir and the chip; placing the chip andthe vapor reservoir in a controlled environment glovebox filled withnitrogen; introducing alkali metal droplets into the vapor reservoir;and sealing a second end of the vapor reservoir by crimping.